74 research outputs found
Spherically symmetric model stellar atmospheres and limb darkening II: limb-darkening laws, gravity-darkening coefficients and angular diameter corrections for FGK dwarf stars
Limb darkening is a fundamental ingredient for interpreting observations of
planetary transits, eclipsing binaries, optical/infrared interferometry and
microlensing events. However, this modeling traditionally represents limb
darkening by a simple law having one or two coefficients that have been derived
from plane-parallel model stellar atmospheres, which has been done by many
researchers. More recently, researchers have gone beyond plane-parallel models
and considered other geometries. We previously studied the limb-darkening
coefficients from spherically symmetric and plane-parallel model stellar
atmospheres for cool giant and supergiant stars, and in this investigation we
apply the same techniques to FGK dwarf stars. We present limb-darkening
coefficients, gravity-darkening coefficients and interferometric angular
diameter corrections from Atlas and SAtlas model stellar atmospheres. We find
that sphericity is important even for dwarf model atmospheres, leading to
significant differences in the predicted coefficients.Comment: 9 pages, 8 figures. Accepted for publication in A&
Revisiting the fundamental properties of Cepheid Polaris using detailed stellar evolution models
Polaris the Cepheid has been observed for centuries, presenting surprises and
changing our view of Cepheids and stellar astrophysics, in general.
Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but
the distance must be measured precisely. The recent debate regarding the
distance to Polaris has raised questions about its role in calibrating the
Leavitt law and even its evolutionary status. In this work, I present new
stellar evolution models of Cepheids to compare with previously measured CNO
abundances, period change and angular diameter. Based on the comparison, I show
that Polaris cannot be evolving along the first crossing of the Cepheid
instability strip and cannot have evolved from a rapidly-rotating main sequence
star. As such, Polaris must also be at least 118 pc away and pulsates in the
first overtone, disagreeing with the recent results of Turner et al. (2013).Comment: 6 pages, 5 figures, resubmitted to A&A after minor revision
Limb Darkening and Planetary Transits II: Intensity profile correction factors for a grid of model stellar atmospheres
The ability to observe extrasolar planets transiting their stars has
profoundly changed our understanding of these planetary systems. However, these
measurements depend on how well we understand the properties of the host star,
such as radius, luminosity and limb darkening. Traditionally, limb darkening is
treated as a parameterization in the analysis, but these simple
parameterizations are not accurate representations of actual center-to-limb
intensity variations (CLIV) to the precision needed for interpreting these
transit observations. This effect leads to systematic errors for the measured
planetary radii and corresponding measured spectral features. We compute
synthetic planetary transits using model stellar atmosphere CLIV and
corresponding best-fit limb-darkening laws for a grid spherically symmetric
model stellar atmospheres. From these light curves we measure the differences
in flux as a function of the star's effective temperature, gravity, mass, and
the inclination of the planet's orbit.Comment: 10 pages, 8 figures, submitted to AAS journals. Comments welcom
Long-term polarization observations of Mira variable stars suggest asymmetric structures
Mira and semi-regular variable stars have been studied for centuries but
continue to be enigmatic. One unsolved mystery is the presence of polarization
from these stars. In particular, we present 40 years of polarization
measurements for the prototype o Ceti and V CVn and find very different
phenomena for each star. The polarization fraction and position angle for Mira
is found to be small and highly variable. On the other hand, the polarization
fraction for V CVn is large and variable, from 2 - 7 %, and its position angle
is approximately constant, suggesting a long-term asymmetric structure. We
suggest a number of potential scenarios to explain these observations.Comment: 2 pages, 1 figure, poster presented at IAU Symposium 301, Precision
Asteroseismology, August 2013, Wroclaw, Polan
Indicators of Mass in Spherical Stellar Atmospheres
Mass is the most important stellar parameter, but it is not directly
observable for a single star. Spherical model stellar atmospheres are
explicitly characterized by their luminosity (), mass () and
radius (), and observations can now determine directly and
. We computed spherical model atmospheres for red giants and for red
supergiants holding and constant at characteristic values
for each type of star but varying , and we searched the predicted flux
spectra and surface-brightness distributions for features that changed with
mass. For both stellar classes we found similar signatures of the star's mass
in both the surface-brightness distribution and the flux spectrum. The spectral
features have been use previously to determine , and now that
the luminosity and radius of a non-binary red giant or red supergiant can be
observed, spherical model stellar atmospheres can be used to determine the
star's mass from currently achievable spectroscopy. The surface-brightness
variations with mass are slightly smaller than can be resolved by current
stellar imaging, but they offer the advantage of being less sensitive to the
detailed chemical composition of the atmosphere.Comment: 24 pages, 9 figure
Period Change and Stellar Evolution of β Cephei Stars
The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change
Period Change and Stellar Evolution of β Cephei Stars
The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change
- …